QGIS in OSGeo4W:

In the installer choose Desktop Express Install and select QGIS to install the latest release.
To get the long term release (that is not also the latest release) choose Advanced Install and select qgis-ltr-full
To get the bleeding-edge development build choose Advanced Install and select qgis-dev-full

Official All-in-one, signed installers

Latest release (richest on features):

Long term release (most stable):

Alternative build

Mac Installer Packages for macOS El Capitan (10.11) and newer.

Installation instructions are in the Read Me on the disk image. These packages use the python.org Python 3, version 3.6, the "macosx10.9" build - other distributions are not supported. Install Python before installing QGIS.

Additional GDAL format plugins and PROJ grids are available at kyngchaos.com.